Delta's ATL-JNB flight is probably one of the most expensive flights for money or points in any class.
They know they're one of two flights non-stop to anywhere in Southern Africa from North America and price themselves accordingly.
Fortunately for them most hunters choose that route because of either convenience, fear of losing firearms, being a Sky team member, using travel agents, having no clue, etc... take your pic.
My suggestion is to use the Google Flights tool which might save you a lot of money and as far as points try to buy two seperate tickets:
One to Europe with money and from Europe to JNB with points within your alliance.You can mix business with economy on different legs as well. This is an option only if you're not flying with firearms.